This invention relates to a signal transmission system, and more particularly to systems in which an optical carrier is used to carry data traffic. A number of optical carriers, each having a different wavelength, can be sent from an optical transmitter to an optical receiver via a light guidexe2x80x94such a technique is termed wavelength division multiplex (WDM), and each optical carrier able to carry traffic is commonly termed a channel.
Optical Add/drop Multiplexers (OADMs) and Optical Cross-connect Switches (OXCs), both of which comprise an optical switching unit, carry multiple traffic signals on the optical channels which are transmitted or received each on a different wavelength via one or a pair of optical fibres. Each OADM and OXC has at least two such wavelength division multiplexed (WDM) ports which can be configured to insert and extract signals from and to tributary ports or to pass signals straight through from one WDM port to another.
A common configuration for a network of OADMs or OXCs is a closed ring because it offers an alternative path for every connection allowing protection against failure of the optical fibre. In order to compensate for the loss of the interconnecting fibres and of the optical components within the OADM or OXC, it contains optical amplifiers. In a system containing amplification and a feedback path, the loop gain must be maintained at less than unity as otherwise unwanted oscillation will occur.
Under fault conditions, the loop gain of the ring may rise above unity, causing instability and malfunction of the ring. The present invention seeks to provide a signal transmission system in which this difficulty is reduced.
According to this invention, a signal transmission system having a plurality of WDM optical carriers, includes a plurality of optical switching units connected in a ring, means present at an optical switching unit for detecting the presence or absence of each individual optical carrier; and means responsive to the detection of the absence of an optical carrier for controlling the ring gain of that optical carrier to be less than unity.
Oscillation in one optical channel, which can be induced by automatic gain control in the absence of an optical carrier, can adversely affect other optical channels due to non-linear effects, power hogging in the amplifiers and crosstalk from the possibly high power oscillation. Even if the loop gain of the ring is less than unity Amplifier Spontaneous Emission (ASE) can accumulate in channels not carrying a normal optical signal to a level which can interfere with adjacent channels.
The optical switching unit may be an OADM or an OXC or other equivalent, and the following discussion of OADMs is applicable to such equivalents.
In OADMs which may be reconfigured (i.e. changes in the channels added, dropped and passed through) some degree of automatic gain control (AGC) is advisable to maintain the power of individual optical channels at a substantially fixed level. Precautions should be taken to ensure that operation of the AGC does not raise the overall gain of any optical channel to a value where oscillation or a disruptive level of ASE accumulation occurs under any conditions. Transient effects can also be a significant problem in linear and ring networks of OADMs resulting from both deliberate changes in the number of channels being carried and during fault conditions. Consider the case when a channel has no optical signal present on it. The AGC function on that channel at each OADM would set itself to maximum gain/minimum loss in an attempt to maintain the power level. When a signal is turned on for that channel the gain at each OADM will be higher than required until the AGC control loops have time to respond. During this transient this signal at each OADM will be raised to an increasingly high level at which it will hog the available power of the optical amplifiers, reducing the signal levels of pre-existing channels and adversely affecting the traffic on them. The transient exists until the AGC control loops settle to their target output powers and optical amplifier control loops accommodate to the increased total output power. While this effect can be partially alleviated by slowly ramping up the power of the new optical signal source this places constraints on the time constants of the various amplifier and AGC control loops. Under fault conditions ramping up the power of the source may not be possible.
These difficulties are reduced by the present invention.